Ion thrusters conventionally include grid units comprising thin metal sheet grids, provided with arrays of holes and molded into convex, or dish, shapes. Such a grid unit characteristically comprises a pair of dish shaped grids mounted one beside the other and spaced a small distance apart. In prior arts methods of making these grid units, the grids of the units, being produced in precisely the same manner and hence having the same orientation, suffer the disadvantage that, when disposed as set forth above, the holes in the two grids are misaligned such that the thrust vectors produced are oriented away from the thruster centerline by an amount which increases with the distance from the center of the grids.
More specifically, presently used methods of making these convex grid units generally entail disposing exposed and developed photoresist patterns for the desired array of holes onto two thin metal sheets. The two thin metal sheets are then laid one on top of the other such that the photoresist patterns for the arrays of holes on the sheets are in alignment. The sheets are secured together, as by tape, and a convex shape is simultaneously imparted to the sheets, such as by hydroforming techniques. The two convex shaped sheets are next subjected to a chemical etching step to form the holes per the photoresist patterns, the sheets then being simultaneously stress relieved, and mounted in the ion thruster apparatus, the inner grid being spaced a small distance from the outer grid. With this method of forming the grid units, the holes of the outer grid are displaced in relation to the corresponding holes of the inner grid by a small distance, x, measured in the plane of the tangents to the holes in a direction toward the centerline of the grid.
In operation, because of this displacement in the alignment of the holes of the two grids, the ions being thrust through the grids are deflected along paths which curve away from the centerline of the grid. Thus, a significant portion of the available thrust imparted to the ions is lost due to this directional misguidance of the ions.